Primary side control is widely applied in conventional isolated switching mode power supplies. FIG. 1 schematically shows a prior art isolated switching mode power supply 10. As shown in FIG. 1, a transformer T1 comprising a primary winding Lp, a secondary winding Ls and a third winding Lt is applied as energy storage element in the power supply 10. A power switch integrated in primary chip IC1 is coupled to the primary winding Lp via terminal “D”. The power switch is controlled to be ON/OFF so as to store energy in the primary winding Lp or to transfer energy from the primary winding Lp to the secondary winding Ls. Then the secondary winding Ls transfers the energy to the output capacitor Co to generate an output voltage Vo. In FIG. 1, the power switch and a control circuit controlling the power switch are integrated in chip IC1. The chip IC1 has a feedback terminal FB configured to receive the output voltage Vo via an OPTO-coupler D0. The control circuit controls the power switch based on the output voltage Vo and a current flowing through the primary winding Lp.
As can be seen from the above description, in order to maintain the proper work of the power supply 10, the opto-coupler D0 should keep working during the normal operation. Persons of ordinary skill in the art should know that the power consumption of the opto-coupler D0 and a resistor R3 coupled to the opto-coupler D0 are constant either under light load or heavy load. Thus the power consumption of the opto-coupler D0 and the resistor R3 accounts for a large proportion of the whole power consumption of the power supply 10 when the load is light, especially when there is no load.